1. Field of the Invention
The present invention relates generally to high-power microwave amplifiers and oscillators.
2. Description of the Related Art
A plasma-assisted high-power microwave generator was disclosed in U.S. Pat. No. 4,912,367 issued Mar. 27, 1990 in the name of Robert W. Schumacher et al. and assigned to Hughes Aircraft Company, the assignee of the present invention. A preferred embodiment in accordance with that patent included a plasma-cathode electron gun coupled to a gas-filled, slow-wave structure (SWS) in the form of a rippled-wall waveguide.
In particular, the electron gun incorporated a plasma cathode in the form of a hollow enclosure filled with a low pressure ionizable gas, e.g., helium or hydrogen. A keep-alive anode was extended into the enclosure and biased to maintain a low current discharge through the gas. Negative pulses applied across the gas then produced a pulsed plasma of electrons and positive ions. A grid and an anode spaced therefrom were positioned adjacent an enclosure outlet. Beam voltage impressed across the anode and grid extracted an electron beam from the plasma and injected it into the tippled-wall waveguide.
Passage of the electron beam through the ionizable waveguide gas produced ions that neutralized the beam and prevented space charge blowup, i.e., a magnetic confining force was produced by the axial beam current which produced an azimuthal magnetic field. This field acted back upon the electron beam to generate a radially inward-directed force thereupon.
The rippled-wall waveguide acted as a slow wave structure to reduce the phase velocity of the electromagnetic waveguide mode so as to match the speed of the electron beam which drifted at less than the speed of light. Space-charge waves on the beam were then resonantly coupled to waveguide modes to transfer energy from the electron beam to a microwave signal which could be coupled to space through an output horn antenna.
Microwave generators in accordance with this structure are capable of high-power, long-pulse radiation, e.g., approximately 1 MW and 100 microseconds, and this is accomplished with a system that neutralizes electron beam space-charge blowup without the use of externally generated magnetic fields. Although some conventional microwave sources, e.g., state-of-the-art klystrons, can achieve these peak power levels and pulse widths, they typically do so with the aid of beam controlling external magnetic fields. These fields are established with surrounding magnetic structures and attendant power supplies that increase the generator size and weight.
However, the resultant microwave signal in the rippled-wall waveguide propagates with the cylindrically symmetric TM01 electromagnetic mode and the waveguide operates near the cutoff frequency of this mode. Because cutoff frequency is inversely proportional to the SWS radius, this property of the rippled-wall waveguide causes the structure to become undesirably large as the operating frequency is reduced to the lower microwave frequencies, e.g., approximately 25 centimeters in diameter and several meters long at 1 GHz.
In addition, the rippled-wall waveguide generator performance is characterized by a small bandwidth and a frequency that varies with beam voltage only in discrete steps with reduced power output between these steps. Finally, the TM01 propagation mode provides a microwave output having an axial null which is difficult to couple to conventional circular or rectangular waveguides. This tippled-wall waveguide feature has typically dictated the use of large, complicated mode converters.
The rippled-wall waveguide is but one example of a slow-wave structure that reduces the electromagnetic wave velocity so that it can interact with an electron beam. Other examples include helixes and coupled cavities. When a helix is used as the slow-wave structure in a microwave tube (commonly called a helix traveling-wave tube), the electron beam is typically controlled to flow through the helix by magnetic focusing structures that envelope the tube. The electron beam is usually formed by a heated cathode which shares the tube interior with the helix and the tube interior is maintained at a high vacuum.
The use of helix slow-wave structures is typically limited to applications where the average power is below 10 kW because of temperature buildup in the helix. In addition, terminations at each end of the helix are generally supported by electrical dielectric structures which are susceptible to arcing when the peak-power exceeds 100 kW. A variety of references describe helix traveling-wave tubes in detail, e.g., Samuel Y. Liao, Microwave Devices and Circuits, Prentice Hall, Englewood Cliffs, 1990, pp. 382-398.
In contrast with the structures described above, some plasma-assisted microwave generators operate by directing two counter-propagating electron beams into a plasma filled waveguide structure. Such structures inherently become more complex and voluminous since two separate electron beam gun structures and attendant coupling with the plasma filled waveguide are typically required. An exemplary generator of this type is described in U.S. Pat. No. 4,916,361 which issued Apr. 10, 1990 in the name of Robert W. Schumacher et al. and was assigned to Hughes Aircraft Company, the assignee of the present invention.